- Type check operator: is
- Type cast operator: as? or as!
-
Defining a Class Hierarchy for Type Casting
class Person {
var name: String
init(name: String){
self.name = name
}
}class Teacher: Person { var class: String init(name: String, class: String){ self.class = class super.init(name) } } class Student: Person { var studentId: Int init(name: String, studentId: Int){ self.studentId = studentId super.init(name) } } //campus is of type Person let campus = [Teacher("Ted", "Math"), Teacher("Ruth", "English"), Teacher("Paul", "Nature"), Student("Amy", 001), Student("Andy", 002)] Checking Type
for person in campus {
if person is Teacher {
print("(person.name) is a teacher.")
}
else if person is Student {
print("(person.name) is a student.")
}
}-
Down Casting
1.as? return an optional value
2.as! will trigger runtime error of cast fail
3.switch case use as(forced cast operator), not as?//optional type cast for person in campus { if let teacher = person as? Teacher { print("\(teacher.name) is a \(teacher.class) teacher.") } else if let student = person as? Student { print("\(student.name)'s ID is \(student.studentId).") } } //forced type cast let teachers: [AnyObject] = [Teacher("Ted", "Math"), Teacher("Ruth", "English"), Teacher("Paul", "Nature")] for obj in teachers { let teacher = obj as! Teacher print("\(teacher.name) is a \(teacher.class) teacher.") }
The cases of a switch statement use the forced version of the type cast operator (as, not as?) to check and cast to a specific type. This check is always safe within the context of a switch case statement.
